This invention relates, generally, to absorption refrigeration, and more specifically to improved apparatus for generating a concentrated absorbent fluid.
Generally, an absorption refrigeration system includes a low pressure side having an evaporator and an absorber and a high pressure side having a generator and a condenser, and uses an absorbent fluid (usually lithium bromide) and a refrigerant fluid (usually water). Absorbent fluid is located in the absorber, refrigerant fluid is located in the evaporator, and the absorber and evaporator are connected together so that refrigerant vapor can pass from the evaporator to the absorber. The absorbent fluid has an affinity for refrigerant vapor and absorbs refrigerant vapor that has passed from the evaporator to the absorber. This absorption produces a pressure drop which helps to maintain the pressure difference between the high and low pressure sides of the system, and it also produces a slightly lower pressure in the absorber than in the evaporator. The pressure difference between the absorber and the evaporator allows more refrigerant vapor to pass from the evaporator to the absorber which permits more refrigerant fluid in the evaporator to evaporate. The refrigerant that remains in the evaporator is thus cooled by evaporation. To take advantage of this refrigeration effect, a heat exchanging coil is positioned in the evaporator and connected to a refrigeration load as part of a closed loop fluid circuit. A heat transfer fluid is circulated through the circuit. The refrigerant in the evaporator absorbs heat from this fluid as the fluid passes through the heat exchanging coil positioned in the evaporator. The fluid, then, absorbs heat from the refrigeration load.
As the absorbent fluid absorbs refrigerant vapor, it becomes diluted by the refrigerant and its affinity for refrigerant vapor decreases. In order to separate the absorbed refrigerant from the absorbing fluid and thereby obtain a concentrated absorbent fluid, the diluted solution of refrigerant and absorbent fluid is pumped from the absorber to the generator. A heat exchanger is positioned in the generator and a heated fluid such as steam or hot water is circulated through the heat exchanger. Conventionally, the liquid, dilute solution is collected in the generator so that the solution floods the heat exchanger. Heat is transferred from the heat exchanger to the solution and this heat vaporizes, or "boils off," refrigerant that had been absorbed by the absorbent fluid. The absorbent fluid is, thus, concentrated and the concentrated absorbent fluid then flows from the high pressure generator back to the low pressure absorber. The refrigerant vapor that was evaporated from the solution passes to the condenser where it is condensed by a cooling liquid flowing through a heat exchanger located in the condenser. This cooling liquid maintains the condenser at a slightly lower temperature than the generator. This results in the pressure in the condenser being slightly lower than the pressure in the generator. Due to this fact, the refrigerant vapor flows naturally from the generator to the condenser. After being condensed, the refrigerant flows from the high pressure condenser to the low pressure evaporator where a new cycle can begin.
The heat needed to vaporize refrigerant from the dilute solution of refrigerant fluid and absorbent fluid; that is, to generate a concentrated absorbent fluid, can be provided by any suitable energy source such as an oil-fired steam generator. In absorption refrigeration systems that use this or other traditional sources of heat, the fluid that is passed through the heat exchanger in the generator is raised to about 250.degree. F. before entering the heat exchanger. Recently, much attention has been directed toward using low temperature energy sources to generate a concentrated absorbent fluid. For example, consideration has been given to solar energy, geothermal energy, and the waste heat produced by many manufacturing processes. These low energy sources usually cannot provide a working fluid that has a temperature greater than 200.degree. F. and often the fluid has a temperature of only 170.degree. F. or 180.degree. F.
It has been learned that, in an absorption refrigeration system which utilizes a low temperature energy source, better results can be obtained if the dilute solution, instead of being collected in the generator and flooding the heat exchanger therein, is sprayed over and through the heat exchanger of the generator. Heat is transferred from the individual heat exchange tubes of the heat exchanger to the solution, and refrigerant evaporates and is separated from the solution. As the solution flows down through the heat exchanger, heat is continuously transferred to the solution and the amount of refrigerant that evaporates from the solution increases. That is, only a small amount of refrigerant may be evaporated from the dilute solution near the top of the heat exchanger, but a relatively large amount of refrigerant may be boiled off from the solution in the lower portion of the heat exchanger.
With prior art spray generators, various factors adversely affect the rate of heat transfer from the heat exchange tubes to the solution flowing over the tubes. For example, there may be poor liquid distribution throughout the heat exchanger. Ideally, there should be a uniform liquid flow rate per unit area of heat exchange tube surface throughout the entire heat exchanger. Typically, in prior art spray generators, the liquid flow rate per unit area of heat exchange tube surface has varied through the heat exchanger, being somewhat higher near the top and slightly lower near the bottom. One reason for this difference, among others, is that the liquid volume decreases as the solution flows through the heat exchanger due to the fact that refrigerant is being separated from the liquid solution and evaporated.
This liquid distribution problem is exacerbated by the refrigerant vapor that is evaporated from the solution in the heat exchanger. When the refrigerant becomes a vapor, it tends to leave the downward liquid flow path and flow directly out of the heat exchanger toward the lower pressure condenser. The heat exchange tubes of the heat exchanger often interfere with this tendency by blocking the vapor inside the heat exchanger. Inside the heat exchanger, the refrigerant vapor tends to force the liquid solution of absorbent and refrigerant fluid away from the surfaces of the heat exchange tubes and to cover the tubes with a vapor film. This "blow-off" factor hinders the transfer of heat from the heat exchange tubes to the solution, and it may become a severe problem near the bottom of the heat exchanger where there may be a significant amount of refrigerant vapor. In accordance with the present invention, it has been found that by varying the spacing between the heat exchange tubes of the heat exchanger in the generator so that the tubes are closely spaced near the top of the heat exchanger and more widely spaced near the bottom of the heat exchanger, vapor "blow-off" can be eliminated, liquid distribution can be improved, and the performance of the spray generator is greatly improved.
Heat exchangers having variably spaced tubes are disclosed in U.S. Pat. No. 3,316,727, and for this reason this patent appears to be the prior art which is most relevant to the present invention. This patent shows a heat exchanger comprised of a multiplicity of variably spaced heat exchange tubes in the absorber and evaporator of an absorption refrigeration system, but does not suggest using such a heat exchanger in the generator of the system. In fact, the above-named patent does not use any type of heat exchanger having a heated fluid flow therethrough to generate a concentrated solution. Instead, a plurality of fire tubes are utilized to separate refrigerant from the dilute solution. Thus, the system disclosed in the U.S. Pat. No. 3,516,727 is not able to take advantage of a relatively low cost energy source such as solar energy to generate a concentrated solution. The present invention, as discussed in greater detail below, is able to so utilize such an energy source. Accordingly, it is felt that the invention disclosed herein is patentably distinguished from the above-mentioned reference patent.
Furthermore, the solution flowing through the heat exchanger in the generator of the present invention flows in the direction in which the tube spacing increases. On the other hand, the U.S. Pat. No. 3,316,727 shows solution flowing through the variably spaced heat exchange tubes in the direction in which the tube spacing decreases--exactly the opposite of the present invention. Thus, the arrangement disclosed in the above-identified patent would be ineffectual in solving the problems such as vapor "blow-off" and poor liquid distribution in the spray generator of an absorption refrigeration system, which are substantially resolved by the present invention. This further supports the patentability of the present invention.
Moreover, the present invention includes a baffle covering the top, closely spaced heat exchange tubes to prevent liquid solution from splashing out of the heat exchanger off of the top tubes. While the U.S. Pat. No. 3,316,727 discloses a baffle, its purpose is simply, referring to line 75 of column 4 and lines 1 and 2 of column 5 of the above-named patent, to direct fluid from the spray nozzles positioned above the heat exchanger onto the individual tubes of the heat exchanger. The baffle disclosed in the U.S. Pat. No. 3,316,727 is not capable of preventing the fluid loss which the baffle of the present invention can prevent; that is, the fluid loss due to solution splashing off of the top tubes. For this reason and the other reasons advanced above, as well as for the reasons discussed below, it is felt that the present invention patentably distinguishes U.S. Pat. No. 3,316,727.